Bernhard,
@Dariusz: the suggested tool is only working without acpi. Without acpi the cores stay disabled. With acpi4.bsd (os4krnl) or with acpi.psd (ibm kernel) the cores disabled by setproc.exe will be enabled immediatly.
So a couple of things I need to point out, I should have actually done so to start off with.
1) hyperthreading
Per the official ACPI read me, the OS/2 drivers do not support hyperthreading...the '/CPUMASK' is meant to shut off hyperthreading in case the actual system BIOS does NOT give you the ability to do so. Since we are talking OS4 here I'm thinking this is not applicable, but for what it's worth I thought I should mention it.
2) setproc
As it turns out given the last couple of releases of the ACPI drivers, when used with the normal OS2 kernel and the ACPI Power Daemon, there is indeed a problem with forcing any particular core into an OFF state. In my case (desktop hardware config, not a laptop) I actually disable the Power Manager feature of the ACPI Daemon, so the Power Manager does not work at all.
If you do not disable the Power Manager function the ACPI Daemon will force the core into it's own chosing of either OFF or ON state, most likely ignoring the selection you had made. This makes sense but happens to be a non-viable configuration for my needs, so Power Manager is permanently shut off and given that my hardware is ON 24x7 that's just fine.